An amorphous silicon semiconductor film obtained by glow discharge decomposition or photo-decomposition of a silicon compound has an excellent ability to convert light into an electrical energy, and is utilized as a photovoltaic device. Utilization of it not only in everyday instruments such as electronic portable calculators but also in solar cells for power generation has been considered. For this purpose, it is necessary to produce solar cells of a large area at low cost. Amorphous silicon-type solar cells can be relatively easily increased in area, and research is under way for increasing their area.
Some problems arise in uniformly forming a semiconductor film of high performance at a high film-forming speed in a film-forming apparatus including conventional parallel plate electrodes of the capacitive coupling type. Firstly, in this apparatus, a substrate for forming a film thereon is positioned between an electrode to which a high frequency is applied (high-frequency application electrode) and an earth electrode. The uniformity of the film in this case cannot be obtained unless the uniformity of glow discharge is secured within the plane of the high-frequency application electrode. Secondly, when a film is to be formed on a substrate of a large area, the area of the high-frequency application electrode should naturally be made larger than that of the substrate. However, in an electrode of a large area, a skin effect inherent to a high-frequency current is produced and the high-frequency current cannot be effectively and uniformly introduced. Furthermore, the aforesaid skin effect and an end effect based on the line of electric force cause glow discharge to occur strongly in the peripheral part of the high-frequency application electrode, and the speed of film formation becomes non-uniform. Moreover, the properties of the resulting thin film are also become non-uniform. In particular, under high-speed film-forming conditions, glow discharge becomes stronger in the peripheral part of the high-frequency application electrode, and such problems become more outstanding.